AIM:   The purpose of this section is to clear up any confusion that might exist between these terms

Definitions 

• IMC (Instrument Meteorological Conditions): Weather conditions precluding flight under the VMC
• IFR (Instrument Flight Rules):  A set of rules for operating under Instrument Flight Rules (see below)
• VFR (Visual Flight Rules):  A set of rules for operating under Visual Flight Rules (see below)
• VMC (Visual Meteorological Conditions):  Weather permitting flight in accordance with the VFR - A set of prescribed minimum weather conditions under which flight may proceed in accordance with the VFR.

NOTE: 

Flights can only be carried out under VFR or IFR (see Special VFR later).   Weather conditions can only be either VMC or IMC

IFR Flight

Firstly it is important to understand that anyone can fly under IFR in any weather conditions. The IFR are merely a set of rules which you have decided to follow during the flight. You do not (necessarily) need an instrument or IMC rating, and it can be a glorious CAVOK day. However you must follow the Instrument Flight Rules as set out below:

• All Flights conducted under the IFR must comply with ANO Rule of the Air 29, which concerns minimum height. You must not fly at a height less than 1000’ above the highest obstacle within a distance of 5 Nm of the aircraft. There are exceptions for take off and landing, and for aircraft below 3000’ MSL.
• All flight conducted under the IFR Outside Controlled Airspace must comply with ANO Rule of the Air 30, which concerns quadrantal & semi-circular rules, and altimeter setting. All level flights above 3000’ or the transition altitude must set 1013mb and use quadrantal cruising levels. Again certain exceptions apply.
• All flight conducted under the IFR Within Controlled Airspace must comply with ANO Rule of the Air 31, which concerns ATC clearances and flight plans. A flight plan must be filed for an IFR flight (possibly an airborne one), and a clearance must be obtained for the flight. All ATC clearances must be followed, and the flight plan must be cancelled after flight.
• All flight conducted under the IFR Within Controlled Airspace must comply with ANO Rule of the Air 32, which concerns position reporting.  The aircraft commander must make position reports when requested.

 So in summary for IFR flight:  Outside CAS:  Rules 29 & 30   Inside CAS:  Rules 29, 31 & 32

VFR Flight

Flight under the IFR can be very restrictive, particularly the minimum height rule. However in certain minimum weather conditions (the VMC), we may choose to operate our flight under the Visual Flight Rules. If we decide to operate under the VFR, then we must follow those rules: 

• All flight conducted under the VFR Outside Controlled Airspace must comply with ANO Rule of the Air 26, which concerns minimum in flight visibility and distance from cloud.
• All flight conducted under the VFR within Controlled Airspace must comply with ANO Rule of the Air 25, which concerns minimum in flight visibility and distance from cloud.
• All flight conducted under the VFR within Controlled Airspace must also comply with ANO Rule of the Air 27, which concerns flight plans and clearances. This deals with the requirement to file a flight plan before a VFR flight in Class D (or B & C) airspace. This can normally be achieved by making an airborne call.

 In summary for VFR flight:      Outside CAS: Rule 26, Inside CAS: Rules 25 & 27

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VMC

These conditions are as follows:

·        AT & ABOVE FL100  Class B,D,E,F,G:  8km visibility and 1500m Horizontally & 1000' vertically from cloud

·        BELOW FL100  Class D,E,F,G:  5km visibility and 1500m Horizontally & 1000’ vertically from cloud

·        AT OR BELOW 3000’   Class F,G:  As above or

·        at speeds > 140kts:   5km visibility and Clear of cloud and in sight of the surface   

·        at speeds 140kts or less:  1500m visibility and Clear of cloud and in sight of the surface

Helicopters have special rules not considered here.

It is very important to understand that these minima stated above are the minimum allowed for that airspace type. It does not mean that all pilots can fly a Cessna 152 at 2000’ in Class G airspace in a visibility of 1500m. 

In addition to the airspace minima, each licence has its own (often more restrictive minima).

For example, a PPL holder with no IMC rating is restricted to an absolute minimum visibility of 3km.

IMC Weather

Any weather conditions poorer than those stated for VMC are by definition IMC, and as such flight must be conducted under the IFR. In such a case, an IMC or Instrument rating would be needed.

Choice of IFR or VFR

A pilot is at liberty to choose either IFR or VFR for a flight as long as the appropriate rules are followed.  Various factors determine the most appropriate choice.  Some points to consider:

• Flight at night must be conducted under IFR unless in a control zone when IFR or special VFR may be used.
• Flight in Class A airspace cannot be conducted under VFR. Only under IFR (or Special VFR in the case of a control zone eg Jersey or Heathrow)
• Flight under IFR in Controlled airspace in circumstances requiring compliance with the IFR (eg Class A, such as airways), require the commander to hold an instrument rating.  Not all controlled airspace requires such compliance (Class B,D,E)

Advantages  of IFR

• Terrain and obstacle clearance
• Traffic separation due to quadrantal levels

 Disadvantages of IFR

• Can be restrictive due to minimum height rule

 Advantages of VFR

• No need for flight plans or clearances
• Can operate at lower levels

 Disadvantages of VFR

• Weather conditions may preclude its use

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 Special VFR

Sometimes due to weather, or at night, a pilot without an instrument rating or IMC rating might be unable to fly within a control zone (Class A, D or E) to reach an airport in that zone.  In such circumstances, ATC may authorise a Special VFR (SVFR) Clearance to the pilot to enable him to depart or arrive at the airport.  Certain considerations apply:

• Normally restricted to a maximum AUW of 5700 kgs
• Clearance must be obtained prior to zone entry or take off
• ATC clearances and instructions must be complied with
• Weather minima: Flight visibility 10km, clear of cloud and in sight of the surface unless an IMC (3km) or Instrument rating (1800m)
• Requirements of ANO Rule 5 (1500’ above highest obstacle within 600m) is waived, but you must still be able to land clear in the event of an engine failure.
• Can only be used in Control zones, not airways.

The IMC Rating

The IMC rating allows the holder to fly in weather below the minima stated for VMC, with the following provisions:

• No flight in control zones under Special VFR in a flight visibility less than 3km
• For take off and landing minimum flight visibility below cloud of 1800m
• It can only be used in UK airspace
• It cannot be used in controlled airspace requiring compliance with IFR (eg Class A airspace)
• The holder may operate under IFR in IMC in Class D, E, F or G airspace

The Instrument Rating

The instrument rating gives the holder:

• All benefits of the IMC rating
• Flights under IFR in controlled airspace in circumstances requiring compliance with the IFR (eg Class A, such as airways)

Flight at Night

Flight at night in the UK can only be made under IFR or SVFR.